1. Field of the Invention
The present invention relates to video signal processing for converting an interlaced-scanning video signal into a progressive-scanning video signal.
2. Description of the Related Art
General video processing apparatuses handle interlaced-scanning video signals. In case of an NTSC (National Television System Committee) system, since one frame is interlaced-scanned by representing it using 525 scanning lines, one field is defined by 262.5 scanning lines. In this case, this value has a fraction of 0.5 lines below the decimal point, and this indicates that when the scanning position returns to the upper left position of a screen to make the next scan after one scan, the scan start position is shifted by the 0.5 scanning lines. As a result, the positions of scanning lines of the first scan and those of the second scan are shifted to display 525 scanning lines by combining the two scans. In this manner, fields are images which are alternately shifted in the vertical direction. In general, an even-numbered field is called an even field, and an odd-numbered field is called an odd field.
On the other hand, as a scanning method different from the interlaced scan, a progressive scan is known. The progressive scan is a system for scanning in the arrangement order of pixels. For example, the progressive scan is made in a plasma display panel (PDP), liquid crystal display (LCD), and image display panel using light-emitting diodes (LEDs) (to be referred to as an LED panel hereinafter). Therefore, in order to display an interlaced-scanning video on an image display apparatus such as a PDP, LCD, or LED panel which makes the progressive scan, processing for converting an interlaced-scanning video signal into a progressive-scanning video signal is required. This processing is generally called IP conversion (interlace to progressive conversion).
Various methods of the IP conversion are known. Recently, in order to enhance image quality, motion adaptive IP conversion is used. With this motion adaptive IP conversion, a motion amount of an object is detected from the difference between pixel data between fields, and line data is adaptively generated according to the motion amount.
The motion adaptive IP conversion executes intra-field interpolation for a moving object, and inter-field interpolation for a still object. More specifically, an image of a field, line data of which is to be generated (to be referred to as a field of interest hereinafter) is interpolated to generate image data suited to a moving picture (to be referred to as interpolation data for a moving picture hereinafter). Also, images of two fields including the field of interest are interpolated to generate image data suited to a still picture (to be referred to as interpolation data for a still picture hereinafter). The interpolation data for a moving picture and that for a still picture are adaptively mixed based on changes in pixel included in a field, thereby generating line data.
The IP conversion which generates image data by mixing the interpolation data for a moving picture and that for a still picture is classified into field difference type motion adaptive IP conversion and frame difference type motion adaptive IP conversion, depending on the factor based on which the mixture ratio of these data is decided.
The field difference type motion adaptive IP conversion decides a mixture ratio based on the difference (to be referred to as a difference between fields) between a pixel to be interpolated (to be referred to as an interpolating pixel hereinafter) of the field of interest and a corresponding pixel of one of fields before and after the field of interest (to be referred to adjacent fields hereinafter). When the difference between fields is large, a high mixture ratio of the interpolation data for a moving picture is set; otherwise, a high mixture ratio of the interpolation data for a still picture is set.
On the other hand, the frame difference type motion adaptive IP conversion decides a mixture ratio based on the differences (to be referred to as differences between frames) between the interpolating pixel of the frame of interest and corresponding pixels of frames before and after the frame of interest (to be referred to as adjacent frames hereinafter). When the differences between frames are large, a high mixture ratio is set for the interpolation data for a moving picture; otherwise, a high mixture ratio is set for the interpolation data for a still picture.
As a merit of these field difference type motion adaptive IP conversion and frame difference type motion adaptive IP conversion, the number of fields required to implement the IP conversion is small. That is, the number of fields required for the field difference type motion adaptive IP conversion is 2, and that required for the frame difference type motion adaptive IP conversion is 3. As a consequence, the memory size of a field memory required to implement the IP conversion can be reduced, thus allowing inexpensive implementation.
Since the field difference type motion adaptive IP conversion uses the field of interest and adjacent fields, it is characterized in that the time resolution is higher than the frame difference type motion adaptive IP conversion, and high-precision IP conversion can be attained in a time direction.
In the motion adaptive IP conversion, when the mixture ratio is decided assuming a moving picture, interpolation processing for a moving picture that executes intra-field interpolation lowers the resolution of an image in the vertical direction. When a moving picture is determined as a still picture by mistake, a video for one frame is generated from data of two different fields according to motion, thus considerably degrading image quality (for example, the edge of an object suffers jaggy, horizontal stripes stand out, an object doubles in some cases, and so forth). The field difference type motion adaptive IP conversion inherits such characteristics of motion adaptive IP conversion processing.